Generally, a mobile communication system is operated under a multipath environment where radio wave reaches a reception side through various propagation paths. Under such a multipath environment, a signal that arrives later interferes with a signal of the present, producing characteristic degradation which is called inter-symbol interference.
However, a transmission method that employs an orthogonal frequency division multiplexing (called the OFDM hereinafter, Orthogonal Frequency Division Multiplexing) as a modulation method can realize a high-speed transmission, without causing characteristic degradation by the inter-symbol interference, even if it is under a multipath environment.
This is because the influence of the inter-symbol interference is alleviated by long symbols used by the OFDM transmission in which a wideband signal is segmented and transmitted in parallel using a plurality of subcarriers that are orthogonal to each other. Against this background, studies have been made on mobile communication systems that use the OFDM as the modulation method.
As an example of a mobile communication system that uses the OFDM as the modulation method, “Performance of an OFDM-TDMA Mobile Communication System” (1593 H. Rohling, R. Grunheid:Proc.of IEEE VTC 1996, and vol. 3, pp. 1589-1996), for example, uses the OFDM as the modulation method for the signal transmission and examines the time division multiple access (TDMA herein below, Time Division Multiple Access:) for communication between a base station and each mobile station.
In this study, it is shown that communication quality will be enhanced by adaptively assigning signals transmitted to mobile stations to subcarriers according to subcarrier reception conditions at the mobile station, utilizing the characteristics of the OFDM that conducts a transmission with a plurality of subcarriers.
Further, a similar study has been made in “Performance Comparison of Different Multiple Access Methods Schemes for the Downlink of an OFDM Communication System” (Proc.of IEEE VTC 1997 pp. 1365-1369, 1997).
In this study, an example of a frame structuring that takes synchronization signal, control signal, and information signal into consideration is shown for application to an OFDM transmission between the base station and the mobile station.
Further, the mobile communication system experiences a phenomenon called fading that is caused by changes in the relative position of the base station and the mobile station, which affects received signals with amplitude fluctuation and phase fluctuation. Therefore, in order to receive a signal transmitted in the mobile communication system by a coherent detection, it is necessary to estimate the amplitude fluctuation and the phase fluctuation accurately to compensate the fluctuations of the received signal using the estimated values for demodulation.
In order to estimate the amplitude fluctuation and the phase fluctuation of the received signal, there is a method wherein a pilot symbol with known amplitude and phase is multiplexed with a transmission signal and transmitted, which is used to estimate the amplitude fluctuation and the phase fluctuation of the received signal on the receiving end. In the mobile communication system which uses the OFDM as the modulation method, the channel estimation using the pilot symbol is also necessary in order to perform the coherent detection, and various studies have been made.
In “Robust Channel Estimation for OFDM System with Rapid Dispesive Fading Channels” (Y. Li, L. J. Cimini, N. R. Sollenberger.IEEE Transactions on Communications, vol.46, no.7, July 1998), for example, a method using a combination of the channel estimation in the time domain and the channel estimation in the frequency domain is shown.
Further, in “Performance Analysis of an OFDM System Using Data-Aided Channel Estimation” (V. Kaasila:Proc.of IEEE VTC 1999, pp. 2303-2307), a study is made as to a time interval at which the pilot symbol should be multiplexed and how much pilot symbol should be used to estimate the channel fluctuation.
However, the various studies mentioned above are concerned with a proposal and an evaluation of the methods for mainly enhancing transmission quality. In order for a mobile communication system to use an OFDM/TDM (Time Division Multiplex) transmission method for a downlink actually, it is necessary to consider a structuring method of the common control channel for transmitting a control signal between-the base station and the mobile station.
Especially, it is necessary to consider use of the channel of not only in the time domain but also in the frequency domain, because the parallel transmission by two or more subcarriers is performed when the OFDM is used as the modulation method.
Furthermore, the studies concerning the pilot symbol insertion have been performed about how much amount of pilot symbol is required, and how the pilot symbols should be inserted only from the point of the measure to a physical fading phenomenon in an environment of one transmitter and one receiver.